JP2005252783A - Optical transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To secure a predetermined eye mask margin in an optical transmitter without depending on a transmission speed. <P>SOLUTION: The optical transmitter includes a semiconductor laser drive circuit 1, and a semiconductor laser 4 for generating and outputting modulated laser light based on the modulation current of the semiconductor laser drive circuit 1. In the above optical transmitter, a phase difference adjustment circuit 2 is provided in the pre-stage of the semiconductor laser drive circuit 1, so as to reduce a phase difference produced between data signals. The phase difference adjustment circuit 2 includes an impedance control line 5, a resistor element 7 having a parasitic inductance component (or an inductor having a parasitic resistance component), and a DC cutoff capacitor 8. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical transmitter, and more particularly to an optical transmitter provided with a phase difference adjustment circuit for reducing a phase difference generated between transmission data.

  Conventionally, as a technique for suppressing an increase in fall time of an optical transmission signal waveform and suppressing ringing, a damping resistor is connected between the semiconductor laser driving circuit and the semiconductor laser, and an external device connected in parallel to the damping resistor. An embodiment of an optical transmission module provided with an attaching resistor is disclosed (for example, Patent Document 1).

Japanese Patent Laid-Open No. 7-46194 (FIG. 1 etc.)

  However, in the optical transmission module according to the prior art disclosed in Patent Document 1, since an external resistor is connected in parallel between the semiconductor laser driving circuit and the semiconductor laser, the current amplitude for driving the semiconductor laser is small. There was a disadvantage that it attenuated. In addition, if the transmission speed of the optical signal changes due to the connection of these damping resistors and external resistors, the time constant of the optical signal waveform rises and falls, affecting the relaxation oscillation of the light. However, there is a problem that the eye mask deteriorates by spreading to the eye mask of the optical signal waveform and a predetermined eye mask margin (eye mask margin) cannot be secured.

  Furthermore, according to this conventional technique, the position where the damping resistor or the external resistor is connected is between the semiconductor laser driving circuit and the semiconductor laser, so that high-frequency design is difficult, and multiple reflections and increased jitter are added to the optical transmission waveform. There was also a problem that the risk of affecting was great.

  The present invention has been made in view of the above, and provides an optical transmitter including a phase difference adjustment circuit that ensures a predetermined eye mask margin without depending on the transmission speed and facilitates high-frequency design. It aims to be realized.

  In order to solve the above-described problems and achieve the object, the present invention provides a semiconductor laser driving circuit for shaping a signal waveform of an input data signal, and a modulation laser based on a modulation current output from the semiconductor laser driving circuit. An optical transmitter including a semiconductor laser that generates and outputs light is characterized in that a phase difference adjustment circuit for reducing a phase difference generated between data signals is provided in a preceding stage of the semiconductor laser driving circuit.

  According to the present invention, since the phase difference adjustment circuit is provided in the previous stage of the semiconductor laser driving circuit, mismatch due to the matching circuit on the transmission line, the capacitance component or the inductor component of the transmission line, or the pattern on the substrate It is possible to reduce the phase difference between data based on data transmissions having different transmission speeds caused by the influence of parasitic capacitance components and parasitic inductance components of wires and wires, the frequency characteristics of semiconductor laser light output, and the like.

  The phase difference adjusting circuit according to the present invention reduces mismatches caused by matching circuits on the transmission line and phase differences based on data transmission with different transmission speeds caused by the characteristics of the transmission line. The eye pattern can be improved, and it is easy to ensure a predetermined eye mask margin.

  Embodiments of an optical transmitter including a phase difference adjusting circuit according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

  First, the optical transmitter of this embodiment will be described with reference to FIG. FIG. 1 is a diagram illustrating a configuration of a main part of a phase difference adjustment circuit and an optical transmitter including the phase difference adjustment circuit according to an embodiment of the present invention. In the figure, a differential amplifier circuit 6 is connected to the output side of the impedance control line 3 to which a normal phase signal and a negative phase signal are input, and phase difference adjustment is performed at the positive phase output terminal and the inverted output terminal which are the output terminals. A semiconductor laser driving circuit 1 is connected via an impedance control line 5 provided in the circuit 2. The semiconductor laser 4 is connected to the output side of the semiconductor laser driving circuit 1 through an impedance control line 9 and a damping resistor 10.

  The phase difference adjusting circuit 2 includes the impedance control line 5 described above, phase difference reducing resistors 7a and 7b, and DC blocking capacitors 8a and 8b. When a resistor such as the phase difference reducing resistors 7a and 7b is used in a high frequency circuit, an inductance component called a parasitic inductance must be considered in addition to the original resistance component. For this reason, FIG. 1 also shows this inductance component in addition to the resistance component. In place of the phase difference reducing resistors 7a and 7b, a phase difference reducing inductor may be used. When this inductor is used in a high frequency circuit, in contrast to the resistor, a resistance component called a parasitic resistance must be considered in addition to the original inductance component.

  Further, the phase difference reducing resistor 7a and the DC blocking capacitor 8a constitute a series resonance circuit and are connected between one line of the impedance control line 5 forming a differential line and the high frequency return path. Similarly, the phase difference reducing resistor 7b and the DC blocking capacitor 8b constitute a series resonance circuit, and are connected between the other line of the impedance control line 5 and the high frequency return path.

  Although the impedance control lines 3, 5, and 9 of this embodiment are shown as differential lines through which a normal phase signal and a negative phase signal are transmitted, they are not limited to differential lines. A single-phase line through which a single data signal is transmitted may be used. For example, the impedance control lines 3 and 5 may be differential lines, and the impedance control line 9 may be configured as a single-phase line. In FIG. 1, the series resonance circuit of the phase difference reducing resistor 7 a (7 b) and the DC blocking capacitor 8 a (8 b) is illustrated as being connected to an arbitrary point on the impedance control line 5. The connection is not limited to this, and it may be connected to each output terminal side of the differential amplifier circuit 6 or may be connected to each input terminal side of the semiconductor laser driving circuit 1.

  Next, the operation of the optical transmitter of this embodiment will be described. In FIG. 1, a differential amplifier circuit 6 shapes the waveforms of a normal phase signal and a negative phase signal input via the impedance control line 5, and generates an adjusted positive phase signal and a negative phase signal. In the prior art, these signals are input to the semiconductor laser driving circuit via an impedance control line or the like. On the other hand, in this embodiment, these signals are input to the semiconductor laser drive circuit 1 via the phase difference adjustment circuit 2. The phase difference adjusting circuit 2 connects the impedance control line 5 with a resistor having an inductance component or an inductor having a resistance component in parallel, thereby providing a frequency characteristic to the group delay characteristic of the data signal transmitted through the impedance control line 5. By providing it, it is inserted in order to reduce the phase difference generated between the data signals. Details of the characteristics of the phase difference adjustment circuit 2 will be described later.

  The signal output from the phase difference adjustment circuit 2 is input to the semiconductor laser drive circuit 1. The semiconductor laser drive circuit 1 is composed of, for example, a pair of differential transistors, a constant current source, and the like, and is interposed between an anode and a cathode of the semiconductor laser 4 via an impedance control line 9 and a damping resistor 10 that are differential lines. A high-frequency modulation current based on the data signal is supplied. A bias voltage is applied to the semiconductor laser 4 in advance, and the semiconductor laser 4 outputs a modulated laser beam based on the modulation current supplied from the semiconductor laser driving circuit 1. This modulated laser beam is transmitted via an optical fiber (not shown).

  Each of the impedance control lines 3, 5, and 9 is inserted for impedance matching between input and output. The damping resistor 10 is inserted in order to suppress the distortion of the modulated light based on the ringing phenomenon.

  FIG. 2 is an explanatory diagram for explaining the principle that a phase difference occurs between input data signals. As shown in the upper left side of the figure, the signal bit pattern has a low frequency frequency component such that “111000111... Thus, a signal bit pattern having a high frequency component such that the same bit does not continue such as “10101010...” Is considered. For signal bit patterns with different frequency components, the data depends on the frequency characteristics (group delay characteristics) of the impedance control line, the frequency characteristics of the circuit, the wire length, etc., as shown in the middle of the figure. There is a phase difference between the signals. When these data signals having a phase difference are incident on the semiconductor laser 4, jitter occurs in the optical signal waveform and degrades the optical signal waveform in combination with the frequency response characteristics of the semiconductor laser 4. If this deterioration is seen in the eye pattern, a thick waveform shifted in the time direction of the waveform is drawn, and as a result, the eye mask margin is reduced.

  On the other hand, in order to obtain a sharp signal when the optical signal output from the optical transmitter is converted into an electric signal and the signal is reproduced on the optical receiver side, the eye pattern is set on the optical transmitter side. It is necessary not to enter an eye mask area (an area for determining signal degradation and an area where an eye pattern should not enter). However, when optical modulation is performed over a wide band from a low frequency range to a high frequency range, the eye mask margin (eye pattern margin), which is the margin of the eye pattern, is caused by the variation in the phase difference due to the variation in the signal bit pattern as described above. The gap between the eye mask region and the eye mask region is reduced, and the transmission characteristics of the output optical signal are deteriorated. Therefore, it is necessary to reduce the phase difference accompanying the variation of the signal bit pattern so that the eye pattern does not enter the eye mask region.

FIG. 3 is a diagram showing an equivalent circuit for analyzing the characteristics of the phase difference adjustment circuit 2. In the figure, only an equivalent circuit on one side of the phase difference adjusting circuit 2 shown in FIG. 1 is shown. In the figure, the impedance of a microstrip line used as an impedance control line is Z ₁ and Z ₂ , a resistor R and an inductor L between the impedance Z ₁ , the microstrip line of impedance Z ₂ , and the ground. The series resonance circuit of the capacitor C is connected.

  FIG. 4 is a diagram showing an example of signal transmission characteristics in the analysis model shown in FIG. 3 (10 Gbps). In the figure, a waveform indicated by a solid line is a transmission waveform of a bit pattern of “0011...” (A frequency component of 2.5 GHz is a main component), and a waveform indicated by a broken line is a bit of “0101. It is a transmission waveform of a pattern (a frequency component of 5 GHz is a main component). As shown in the figure, in this example, the bit pattern signal “0011...” Is delayed as compared to the bit pattern signal “0101. The cause of such a phase difference may be the influence of the capacitance component or inductor component of the microstrip line.

  FIG. 5 is a diagram showing the group delay characteristics of the microstrip line used in the analysis model of FIG. As shown in the figure, the delay time is larger at 2.5 GHz (point m1) than at 5 GHz (point m2), and the transmission characteristic shown in FIG. 4 is the group delay characteristic of the microstrip line. It is clear that it depends on

  FIG. 6A is a diagram illustrating a delay time in the transmission characteristics illustrated in FIG. 3, and FIG. 6B is a diagram illustrating a delay time in the transmission characteristics when a predetermined phase difference adjustment is performed. In FIG. 6A, the bit pattern signal “0011...” Is delayed by about 3.5 (psec) from the bit pattern signal “0101. After being performed, as shown in FIG. 6B, the signal of the bit pattern “0101...” Is delayed by about 1.7 (psec) from the signal of the bit pattern “0011. .

  FIG. 7 is a diagram illustrating group delay characteristics in the transmission characteristics illustrated in FIG. Contrary to the group delay characteristics shown in FIG. 5, the delay time value is larger at 5 GHz (point m4) than at 2.5 GHz (point m3), and the transmission characteristics shown in FIG. The results are clearly shown.

  FIG. 8A is a diagram illustrating an example of a transmission characteristic of a signal that transmits a pattern, a wire, or the like on a substrate through which the signal is transmitted. FIG. It is a figure which shows the transmission characteristic at the time of performing a predetermined phase difference adjustment. As in FIGS. 4, 6-1, and 6-2, each figure is a transmission waveform of a bit pattern “0011...” And a broken line is a bit pattern of “0101. It is a transmission waveform. In FIG. 8A, unlike FIG. 6A, the bit pattern signal (5 GHz) of “0101...” Is delayed from the bit pattern signal (2.5 GHz) of “0011. The delay of the high frequency side signal in this way is due to the influence of the parasitic capacitance and parasitic inductance of the pattern and the wire on the substrate. Although detailed numerical values are omitted, each of R, L, and C in the analysis model of FIG. 3 is a predetermined value even when such a high-frequency signal is delayed with respect to a low-frequency signal. By setting to, the phase difference between the high frequency side signal and the low frequency side signal can be reduced as shown in FIG.

  So far, we have described in detail how to reduce the phase difference that occurs between signal data based on the group delay characteristics of the transmission line and the frequency characteristics of the patterns and wires on the transmission board. In addition, other factors such as the frequency characteristics of the optical output of the semiconductor laser 4 must be taken into consideration. The values of R, L, and C in the series resonance circuit configuration of the phase difference adjustment circuit 2 must be determined in consideration of all these factors.

  In this embodiment, the configuration in which the phase difference adjustment circuit 2 is added to the optical transmitter has been described. However, the phase difference adjustment circuit 2 can also be applied to the optical receiver. However, as is apparent from the comparison of the waveforms of FIGS. 6A and 6B, when the phase difference adjustment circuit 2 is inserted, the amplitude level of the signal decreases. Therefore, as in the optical transmitter of this embodiment, the phase difference adjusting circuit 2 is inserted before the semiconductor laser driving circuit 1, and in the optical receiver, it is inserted after the amplifier (for example, preamplifier). Is preferred.

Next, effects when the phase difference adjusting circuit 2 is used will be described. 9 to 10-2 are diagrams illustrating eye pattern measurement results of the optical signal waveform output from the semiconductor laser 4. FIG. More specifically, FIG. 9 is a diagram showing an eye pattern of an optical signal waveform when the phase difference adjustment circuit 2 is not used, while FIGS. 10-1 and 10-2 show the phase difference adjustment circuit 2. It is a figure which shows the eye pattern of the optical signal waveform at the time of using. 9 to 10-2, the output power P ₀ of the semiconductor laser 4 is all 10 mw. Further, in the measurement result shown in FIG. 10-1, the resistance value and the capacitor of the phase difference adjustment circuit 2 are set to R = 15Ω and C = 0.1 μF, respectively. In the measurement result shown in FIG. R = 15Ω and C = 0.1 μF.

  The eye patterns shown in FIGS. 9 to 10-2 are obtained by performing optical-electrical conversion on optical signal waveforms of various signal patterns, passing through bandpass filters, and overwriting the electrical signals. The side and the lower part are the space side, and the horizontal axis indicates time. The figure also shows an eye mask region (shaded portion) defined by the eye mask definition.

  Here, the eye mask region is an index for viewing the performance of the signal waveform. The larger the gap (eye mask margin) between the signal waveform (eye pattern) and the eye mask area, the better the reception sensitivity characteristic is obtained when the signal is received by the optical receiver. In other words, errors are less likely to occur even with weaker power optical signals, and good transmission characteristics can be obtained.

  That is, when receiving an optical signal output from an optical transmitter, converting it to an electrical signal, and reproducing the signal, an eye pattern does not enter the eye mask region in order to obtain a signal with a low error rate. It is necessary to make it.

  However, in the portion in the circle on the right side of the center of the eye pattern shown in FIG. 9, the eye mask margin is about 2%, and the margin for the eye mask definition is small. If the eye mask margin is insufficient, there arises a problem that, for example, the signal waveform cannot satisfy the upper right eye mask in the central portion depending on a change in ambient temperature.

  On the other hand, in FIG. 10A, the eye mask margin is improved to about 5% and the margin for the eye mask definition is increased in the part in the circle on the right side of the center of the eye pattern at the same position as in FIG. ing. In FIG. 10-2, the eye mask margin is about 7% in the circle in the right side of the center of the eye pattern, which is further improved compared to FIG. 10-1.

  As is clear from the above results, the resistance value and capacitor of the series resonance circuit provided in the phase difference adjustment circuit 2 are set to predetermined values, so that the optical signal waveform output from the semiconductor laser 4 is in accordance with the eye mask specification. The margin can be increased.

  As described above, according to the optical transmitter of this embodiment, the phase difference adjustment circuit provided in the previous stage of the semiconductor laser driving circuit is not matched by the matching circuit on the transmission line, or the characteristics of the transmission line. Since the phase difference based on data transmission with different transmission speeds caused by the above can be reduced, the eye pattern of the optical signal waveform can be improved, and a predetermined eye mask margin can be secured.

  As described above, the optical transmitter according to the present invention is useful as an optical transmitter for high-speed digital communication using an optical fiber, and in particular, the transmission speed generated by mismatching of the matching circuit on the transmission line is different. This is suitable when the phase difference between data becomes a problem. Further, the phase difference adjustment circuit provided in the optical transmitter of the present invention can also be applied to an optical receiver.

It is a figure which shows the structure of the principal part of the optical transmitter provided with the phase difference adjustment circuit concerning the embodiment of this invention, and the phase difference adjustment circuit. It is explanatory drawing for demonstrating the principle which a phase difference produces between input data signals. 3 is a diagram showing an equivalent circuit for analyzing the characteristics of the phase difference adjustment circuit 2. FIG. It is a figure which shows an example of the signal transmission characteristic in the analysis model shown in FIG. It is a figure which shows the group delay characteristic of the microstrip line used with the analysis model of FIG. It is a figure which shows the delay time in the transmission characteristic shown in FIG. It is a figure which shows the delay time in the transmission characteristic at the time of performing a predetermined phase difference adjustment. It is a figure which shows the group delay characteristic in the transmission characteristic shown to FIGS. 6-2. It is a figure which shows an example of the transmission characteristic of the signal which transmits the pattern, the wire, etc. on the board | substrate where a signal is transmitted. It is a figure which shows the transmission characteristic at the time of performing predetermined phase difference adjustment in the transmission characteristic shown to FIGS. It is a figure which shows the eye pattern of the optical signal waveform at the time of not using the phase difference adjustment circuit 2. FIG. It is a figure which shows the eye pattern (R = 15 ohm, C = 0.1 micro F) of the optical signal waveform at the time of using the phase difference adjustment circuit 2. FIG. It is a figure which shows the eye pattern (R = 27 ohms, C = 0.1 micro F) of the optical signal waveform at the time of using the phase difference adjustment circuit 2. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor laser drive circuit 2 Phase difference adjustment circuit 3, 5, 9 Impedance control line 4 Semiconductor laser 6 Differential amplifier circuit 7a, 7b Resistance for phase difference reduction 8a, 8b DC blocking capacitor 10 Damping resistor

Claims (6)

In an optical transmitter including a semiconductor laser driving circuit that shapes a signal waveform of an input data signal, and a semiconductor laser that generates and outputs modulated laser light based on a modulation current output from the semiconductor laser driving circuit,
An optical transmitter comprising a phase difference adjustment circuit for reducing a phase difference generated between data signals in a stage preceding the semiconductor laser driving circuit. The phase difference adjusting circuit is
A transmission line for transmitting the data signal to the semiconductor laser driving circuit;
A series resonant circuit connected to the transmission line;
With
The series resonant circuit is:
A resistive element having a parasitic inductance component or an inductor having a parasitic resistance component;
A capacitance element;
The optical transmitter according to claim 1, further comprising:   The optical transmitter according to claim 1, wherein the series resonant circuit is connected to an input side or an output side of the transmission line.   The optical transmitter according to claim 1, wherein the transmission line is a differential line, and the phase difference adjustment circuit is connected to each of the differential lines.   The optical transmitter according to any one of claims 1 to 3, wherein the transmission line is a single-phase line.   The optical transmitter according to claim 1, wherein the transmission line is a micro slip line.

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